JP2005206918A - Magnesium based hydrogen storage material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an Mg based hydrogen storage material, for solving the problem that the hydrogen storage material is expected as an application to fuel storage for a fuel fuel, but, its hydrogen storage/discharge rates are low and hydrogen storage/discharge temperatures are also high, thus its practical use therefor has been difficult, having practical hydrogen storage/discharge temperature, practical hydrogen storage/discharge rates and discharge pressure equal to or above the atmospheric pressure. <P>SOLUTION: The Mg based hydrogen storage material contains Mg and carbon, or Mg, carbon and Ni. At the time when carbon is added to Mg, the hydrogen storage/discharge quantity within practical time increases compared with the case of only Mg, and further, hydrogen dissociation pressure also lowers compacted with the case of only Mg. Further, Ni is added, thereby lowering the hydrogen discharge temperature. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a magnesium (Mg) -based hydrogen storage material, and more specifically to a magnesium-based hydrogen storage material that has excellent hydrogen storage / release capability and can store and release hydrogen at a relatively low temperature and relatively high speed.

In recent years, there has been an active movement to use hydrogen as energy. In particular, when hydrogen is used as a fuel source for a fuel cell, it does not emit harmful exhaust gas, and thus has attracted attention as a fuel suitable for the environment.
As a method of storing hydrogen fuel, a method of storing liquid hydrogen or gaseous hydrogen in a tank and a method of storing hydrogen in a hydrogen storage alloy are known. When stored in liquid hydrogen, the liquefaction temperature is very low, minus 252.8 ° C. Therefore, if a highly heat-insulating container is not used, vaporization (boil-off) will be severe and hydrogen as a fuel will leak slightly. There is difficulty in storage for the period. Moreover, when storing with gaseous hydrogen, gas is compressed and stored to 700 atmospheres or more. For this reason, a container that can withstand ultra-high pressure is required, but it is important to consider sufficient safety. Instead of these, the method of storing hydrogen by storing it in the hydrogen storage alloy has the advantage that the above-mentioned cryogenic temperature and high pressure are not required, but the hydrogen storage amount is as low as less than 3% by weight.

Mg is known as a metal having a large hydrogen storage capacity. Mg metal can theoretically store hydrogen up to 7.6% by weight. However, Mg metal has a slow hydrogen storage rate, and unless the temperature is raised to 340 ° C. or higher, hydrogen cannot be fully stored within the practical time. Further, it is said that a temperature of 340 ° C. or higher is required for hydrogen release. Had the disadvantages. This is because MgH ₂ is difficult to dissociate and the diffusion coefficient of hydrogen is small. If the operation of absorbing and releasing hydrogen into Mg is attempted within a practical time, only a considerably smaller amount than the theoretical hydrogen storage amount can be used.
Known methods for improving the above-mentioned drawbacks of Mg metal include a method of adding I ₂ , Ni and Cu as catalytic elements, a method of plating Ni on the Mg metal surface, and a method of adding 3A group Al, Ga and In. It has been. In addition, a method of mechanical alloying by adding Fe, Ni fine particles, or C (carbon) to Mg metal is known.

As another example of the Mg-based composite material to which the thin film material is applied, a multilayer film of Mg and Pd is disclosed in Patent Document 1. However, this composite material is not practical because it uses rare Pd which is a rare resource.
Further, although Mg-Ni-graphite material used as an electrode material for purposes other than hydrogen storage is described in Non-Patent Document 1, there is no mention of hydrogen storage material.
JP 2002-105576 A Journal of Alloys and Compounds 293-295 (1999) p653-657

As described above, Mg metal as a hydrogen storage material has satisfactory performance in terms of the amount of hydrogen stored, but has not obtained a hydrogen storage / release rate that can be put to practical use. Further, there is a problem that the discharge temperature requires a high temperature of about 340 ° C. If this temperature can be increased to 320 ° C., particularly less than 300 ° C., it can be used as a hydrogen storage material with high practical value. Therefore, there is a demand for a hydrogen storage material that can perform practical hydrogen storage / release at a relatively low temperature without significantly reducing the amount of hydrogen storage.
The present invention has a practical hydrogen dissociation pressure, hydrogen dissociation temperature, and hydrogen absorption / desorption amount, and further has a release pressure of atmospheric pressure or higher, and Mg-based hydrogen capable of absorbing and desorbing hydrogen at a relatively low temperature. An object is to provide an occlusion material and a method for producing the same.

  The present invention relates to a hydrogen storage material characterized by comprising Mg and carbon, and a Mg-based hydrogen storage material characterized by comprising Mg, carbon (C) and nickel (Ni), and sputtering. These manufacturing methods.

The present invention will be described in detail below.
The inventors of the present invention have maintained the hydrogen storage / release capability or the hydrogen storage / release temperature while maintaining the hydrogen storage / release capability that is the greatest obstacle to the practical application of the hydrogen storage material made of Mg-based materials. Various studies have been made to reduce the speed to a practical level in terms of the material added to Mg metal and the manufacturing method, and the present invention has been achieved.
The present inventors have found carbon as an additive material having such characteristics. Carbon added to Mg metal substantially increases the hydrogen storage capacity of Mg metal. In other words, the hydrogen storage capacity of the Mg-carbon-based hydrogen storage material obtained by carbon addition is slightly lower than 7.6 wt%, which is the theoretical hydrogen storage capacity of Mg metal, but the grain growth during the activation treatment is suppressed. In addition, since the hydrogen storage / release speed is much faster, the amount of hydrogen storage / release available is increased. Furthermore, the hydrogen release temperature is lowered to a practical level, and hydrogen storage / release at a relatively low temperature is enabled.
The amount of carbon added is desirably 0.3 to 10% by weight, and if it is less than 0.3% by weight, the effect of carbon addition hardly appears, and if it exceeds 10% by weight, the Mg ratio to the whole decreases, and hydrogen May adversely affect release.

When Ni is added to this Mg—C-based material, the occlusion and desorption are further accelerated, the hydrogen desorption temperature is further lowered, and a more practical hydrogen occlusion material can be obtained. However, when C is not added, the aforementioned grain growth occurs and the maximum occlusion amount decreases. Ni is often used as a hydrogen dissociation catalyst and has a function of improving the hydrogen absorption / release rate. Although it is especially used as an Mg ₂ Ni alloy, the hydrogen storage amount is as low as 3.6% by weight, which is halved compared with 7.6% by weight of Mg metal. Therefore, it is desirable that the amount of Ni added is as small as possible, for example, 1 to 20% by weight.
These hydrogen storage materials according to the present invention usually have a hydrogen dissociation pressure of 0.1 to 0.3 MPa, that is, an atmospheric pressure or more, and the temperature at this time is at least 320 ° C. lower than the hydrogen dissociation temperature 340 ° C. of pure Mg. In some cases, the temperature is lowered to about 260 ° C., and the hydrogen absorption / release amount is at least 5.5 wt% or more.

The phase diagram of Mg and carbon is not known and cannot be produced by normal casting methods. However, it is known that Mg ₂ C ₃ and MgC ₂ are produced as compounds at a reaction temperature of 500 ° C. or higher. Therefore, the Mg-based hydrogen storage material of the present invention can be produced by a dry synthesis method in which vapor synthesis is performed in a gas atmosphere, a thin film formation method such as vapor deposition or sputtering, or a powder synthesis method such as mechanical alloying (MA).
For example, MA is a method in which a mixture of Mg metal powder and carbon powder such as graphite is vigorously stirred by a ball mill to prepare an alloy or a similar composite. However, Mg-based hydrogen storage materials prepared by this method have not achieved satisfactory performances so far.
Sputtering, on the other hand, is a method of energizing an Mg or graphite target, irradiating atoms toward the substrate to form a film on the substrate, and then peeling off the substrate to obtain a desired metal, alloy or composite. Materials mixed or laminated at levels are obtained.

The present invention is an Mg-based hydrogen storage material characterized by comprising Mg and carbon, or an Mg-based hydrogen storage material characterized by comprising Mg, carbon and Ni.
The hydrogen storage material containing Mg and carbon increases the amount of hydrogen storage / release within a practical time, in other words, the hydrogen storage / release rate, compared to the hydrogen storage material of Mg metal alone. Furthermore, the hydrogen dissociation pressure is higher than that in the case of Mg metal. Therefore, the temperature for setting the hydrogen release pressure to at least atmospheric pressure is less than 320 ° C., which is lower than the hydrogen dissociation temperature of pure Mg metal, 340 ° C. In addition, the hydrogen absorption / release amount is at least 5.5 wt% or more.

Although hydrogen storage materials containing Mg, carbon and Ni tend to have a slight decrease in hydrogen release compared to hydrogen storage materials containing Mg and carbon, they can lower the hydrogen release temperature and operate at low temperatures. This is particularly useful in applications where demands are required.
These Mg-based hydrogen storage materials are preferably dry synthesis methods that can produce as fine particles as possible, and the resulting materials can maintain small crystal particles even at the activation temperature, and the atomic rules necessary for hydrogen storage Can be secured, and the amount of hydrogen storage / release increases.

The manufacturing method of Mg type | system | group hydrogen storage material by sputtering of this invention method is demonstrated based on the example of embodiment shown in FIG.
FIG. 1 is a schematic front view of a sputtering apparatus that can be used for producing an Mg-based hydrogen storage material by sputtering.
A suction port 14 connected to an argon gas inlet 13 and a vacuum pump (not shown) is formed in the bottom wall 12 of the sputtering chamber 11, and a rectangular carbon target 15 is installed on the inner surface of the bottom wall 12, The target 15 is grounded via a target power supply 16. A rectangular Ni target 19 and an Mg target 20 are respectively installed on the left and right side walls 17 and 18 continuing to the bottom wall 12, and both the targets 19 and 20 are grounded via target power sources 21 and 22, respectively. .
A regular octagonal rotatable substrate holder 23 is installed in the center of the chamber 11 so as to face in the vertical direction. A total of eight plate-like substrates 24 are fixed to each side of the regular octagon of the holder 23. The holder 23 is configured to rotate.

The inside of the sputtering chamber 11 is evacuated using a vacuum pump, and then argon gas is introduced from the argon gas inlet 13 to keep the inside of the chamber under reduced pressure.
When electric power is supplied from the respective target power sources 16, 21, and 22 so that the carbon target 15, the Ni target 19, and the Mg target 20 have a desired composition while rotating the substrate holder 23, C atoms, Ni atoms, and Mg Atoms are irradiated onto the surface of the substrate 24, and an Mg—C—Ni alloy is formed in layers on the surface of each substrate 24. By peeling this layered alloy from the substrate 24, a desired Mg-based hydrogen storage material can be obtained.

  Next, examples relating to the production of the Mg-based hydrogen storage material according to the present invention will be described, but the examples do not limit the present invention.

[Examples 1-7]
Commercially available Mg metal and graphite plates were molded to 5 inches × 12 inches square and 6 mm thick to form Mg targets and C targets, respectively.
The Mg target and C target were fixed to the wall surface in the sputtering chamber so as to face each other, and a rotatable substrate holder was installed on the chamber floor between the two targets, and the substrate was held by this holder.
The chamber was evacuated using a rotary pump and a cryopump, and then argon gas was introduced to create an argon atmosphere of 0.13 to 4 Pa (about 1 to 30 mtorr). While rotating the substrate holder at 10 to 30 revolutions / minute, the Mg target and C target are energized at 0.2 to 2 kW and simultaneously discharged to deposit Mg and C on the substrate surface to form a substrate. Removal of the Mg-based hydrogen storage alloy was obtained.
The film thickness was adjusted by changing the number of rotations of the substrate holder and the input power to each target, the amount of graphite in Mg was controlled, and Mg—C alloys shown in Examples 1 to 7 in Table 1 were produced. The amount of carbon was determined by the sample combustion method.

As activation treatment, each Mg-based hydrogen storage alloy was heated to 430 ° C. in a vacuum, and then pressurized to 4 MPa with hydrogen at 370 ° C. This vacuum and pressurization operation was repeated 3 cycles.
Thereafter, the hydrogen absorption / release amount of each alloy was measured by adopting the vacuum origin method defined in the pressure composition isotherm measurement method (JIS H7201) by the volume method, and changing the temperature. The equilibrium waiting time at this time was a maximum of 15 minutes. The hydrogen release temperature was determined from the relationship between the temperature and pressure shown in the Van Hof equation, and was defined as the temperature when the hydrogen dissociation pressure was 0.2 MPa. The hydrogen release temperature and the hydrogen release amount were measured for each alloy, and the results are summarized in Table 1. The PCT curve of Example 3 is shown in FIG.

[Examples 8 to 11]
Except that the Ni target was used in addition to the Mg target and the C target, an Mg-based hydrogen storage alloy was produced and activated under the same conditions as in Example 1, and the Mg − shown in Examples 8 to 11 in Table 1 were used. A graphite-Ni alloy was prepared.
For each alloy, the hydrogen release temperature and the hydrogen release amount were measured in the same manner as in Example 1, and the results are summarized in Table 1. The amount of Ni was determined by ICP analysis.

  Further, for the alloy having the lowest hydrogen release temperature obtained in Example 9, the hydrogen storage characteristics and release characteristics (PCT curve) were measured at a predetermined temperature. The PCT curves are shown in the graphs of FIGS. FIG. 3 is a graph showing a normal PCT curve of the Mg-carbon-Ni alloy obtained in Example 9, and FIG. It is the graph which measured the PCT curve precisely in the temperature range in which the plateau pressure of around 2 MPa is obtained.

[Comparative Example 1]
Mg metal was produced under the same conditions as in Example 1 except that only the Mg target was used.
FIG. 5 shows a PCT curve in which the hydrogen release temperature and the hydrogen release amount of this Mg metal were measured in the same manner as in Example 1. The results are summarized in Table 1.

[Comparative Example 2]
An Mg—Ni alloy was produced under the same conditions as in Comparative Example 1 except that the Ni target was added and the Mg target and the Ni target were used.
For this Mg—Ni alloy, the hydrogen release temperature and the hydrogen release amount were measured in the same manner as in Example 1 and shown in FIG. 6. The results are summarized in Table 1.

[Consideration of Examples and Comparative Examples]
In Comparative Example 1, the hydrogen release amount of Mg metal was 5.1% by weight, which was much smaller than the theoretical value, but this was a slow hydrogen release rate, and 5.1% at the set equilibrium waiting time (15 minutes). This is because only% of hydrogen was released. In Comparative Example 2, the hydrogen release temperature was slightly lowered and the hydrogen release rate was increased by adding Ni, but the hydrogen storage / release amount was the same as that of Mg metal and could not be increased.
In the Mg—C alloys of Examples 1 to 7, the hydrogen release temperature decreased from 339 ° C. of Mg metal, and an alloy having a hydrogen release temperature of 287 to 316 ° C. was obtained. Compared with Mg metal, the temperature was lowered by 23 to 52 ° C., and hydrogen storage / release at low temperatures became possible.

Furthermore, the hydrogen release amount of the Mg—C alloys of Examples 1 to 7 was 6.0 to 6.9% by weight, and the decrease amount was slight even compared to 7.6% by weight which is the theoretical storage value of Mg metal. As described above, when the hydrogen release amount within the measurement time is compared, it can be seen that a release rate sufficiently higher than that of Mg metal (5.1 wt%) was obtained.
Next, in the Mg—C—Ni alloys of Examples 8 to 11, it can be seen that the hydrogen release temperature was lower than that of Comparative Examples 1 and 2, and the hydrogen release amount increased. Furthermore, the hydrogen release amount of this Mg—C—Ni alloy is 5.7 to 6.8% by weight, and the overall hydrogen release amount is smaller than that of the Mg—C alloys of Examples 1 to 7, but hydrogen release It can be seen that the temperature was 267 to 312 ° C., and the temperature was further lowered and became practical.

The schematic front view of the sputtering device used for manufacture of Mg type | system | group hydrogen storage material by sputtering. 4 is a graph showing a PCT curve of the Mg-carbon alloy obtained in Example 3. FIG. The graph which shows the PCT curve of the Mg-carbon-Ni alloy obtained in Example 9. The graph which measured the PCT curve precisely in the temperature range in which the plateau pressure of about 0.2 MPa is obtained in consideration of the atmospheric pressure release of the Mg-carbon-Ni alloy obtained in Example 9. 4 is a graph showing a PCT curve of Mg obtained in Comparative Example 1. The graph which shows the PCT curve of the Mg-Ni alloy obtained in Comparative Example 2.

Explanation of symbols

11 Sputtering chamber 13 Argon gas inlet 14 Suction port 15, 19, 20 Target 23 Substrate holder 24 Substrate

Claims (6)

  A magnesium-based hydrogen storage material comprising magnesium and carbon.   The magnesium-based hydrogen storage material according to claim 1, comprising 0.3 to 10% by weight of carbon and the balance of magnesium.   A magnesium-based hydrogen storage material comprising magnesium, carbon and nickel.   The magnesium-based hydrogen storage material according to claim 3, comprising 0.3 to 10% by weight of carbon, 1 to 20% by weight of nickel, and the balance of magnesium.   A method for producing a magnesium-based hydrogen storage material containing magnesium and carbon by sputtering a magnesium target and a carbon target on a substrate surface in an inert gas atmosphere.   A method for producing a magnesium-based hydrogen storage material containing magnesium, carbon, and nickel by sputtering a magnesium target, a carbon target, and a nickel target on the substrate surface in an inert gas atmosphere.

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